The invention pertains to a process for manufacturing filaments from an optically anisotropic spinning solution in which the spinning solution is extruded through spinning orifices grouped in at least one spinning section and in which the extrudates are passed through an inert gas and a coagulation bath in succession.
Such a process is known from Japanese Patent Publication No. 1986-239012, which describes a method of spinning filaments from poly(para-phenylene-terephthalamide), also termed "PPTA", where the filaments are spun through spinning orifices grouped to form a rectangle. The ratio of the rectangle's long side length to its short side length has to be at least 4. In the coagulation bath a hole, also rectangular, is provided beneath the spinning section. Since both the spinning section and the hole in the coagulation bath are rectangular, the bundle of filaments is rectangular also. As a result of this rectangular shape of the bundle, very few vortexes are created in the coagulant, a portion of which is discharged from the coagulation bath together with the filaments. This leads to a substantial reduction of filamentation in the coagulation bath (where the filaments are not yet fully coagulated) and makes it possible to increase the spinning speed.
In the Examples of said Japanese patent specification filaments of good strength are made. This strength is to be attributed first of all to the coagulant's low concentrations of sulfuric acid (0 and 10%) and the wideness, on average, of the spacing of the spinning orifices (the so-called "pitch"). The low acid concentration, which can only be maintained by treating the coagulant and replenishing it, and the large pitch, which makes it necessary to employ a large apparatus in relation to the number of produced filaments, make the described process into an expensive one with a very large waste stream.
Furthermore, at high spinning speeds there will have to be a subatmospheric pressure beneath the coagulation bath, this in order to further accelerate the speed of the coagulant and so reduce the tension in the filaments.
If in the process according to the Japanese patent specification the pitch of the spinning orifices is reduced in order to increase their number (and hence the number of filaments) per unit of area, the filaments in the coagulation bath will stick together at the spinning speeds mentioned, rendering the end product unsuitable for use in the envisaged high-grade applications (e.g., woven fabrics or composite reinforcement).